When using a rechargeable battery pack including a plurality of secondary batteries connected in series as described in Patent Document 1, it is generally known that variation will arise in the quantity of accumulated charge of the batteries while they are being used, and the continued use thereof may cause only certain secondary batteries to become an overcharged state or an overdischarged state, whereby the expected life may become shortened or the secondary batteries may become an unsafe state. Thus, in order to resolve the variation in the state of charge (charging capacity) of the respective secondary batteries, a control method is known where charging is performed until the secondary battery with the smallest quantity of accumulated charge is fully charged and intentionally causing the other secondary batteries to become an overcharged state so as to eliminate any variation in the state of charge in all secondary batteries.
Nevertheless, according to the foregoing control method, in cases where an assembled battery in which a plurality of secondary batteries are connected in series is additionally connected in parallel, if a charging current is supplied to the assembled battery in order to cause the assembled battery with a variation in the state of charge to become an overcharged state, the charging current will also be supplied to an assembled that is connected in parallel to the foregoing assembled battery in which no variation has occurred. Thus, there was an inconvenience in that unnecessary power loss will increase.    Patent Document 1: Japanese Patent Application Laid-open No. H4-75430